This project comprises two twin studies. Project-A is a large-sample (by the standards of psychophysiology), cross-sectional study of the psychophysiological parameters most strongly implicated as substrates of emotional individuality, or affective style. Project-B is a twin study of young children at risk for internalizing problems such as anxiety, social withdrawal, and depression. These two studies help constitute the Center's foci on individual differences, fearful behavior, integration of the study of normal and disordered behavior, and psychophysiological and quantitative genetic substrates of behavior. For the biological variables such as EEG asymmetry, basal cortisol levels, fear-potentiated startle, cardiac psychophysiology, or aspects of amygdala functioning, there is often an implicit expectation that individual variability will have at least a partial genetic basis: Project-A tests this expectation, and also investigates whether there is a common genetic basis for phenotypically associated behavior (such as inhibitory tendencies) and biological substrates (such as frontal EEG asymmetry). Measures in the study include central and autonomic psychophysiological measures (including EEG and impedance cardiography), as well as cortisol and behavioral measures of temperament in 6-8 year-olds. Project-B is the twin study of extreme temperamental groups putatively at risk for internalizing disorders of childhood. After identification of the probands, we will assess them during a home visit with instruments designed to characterize their internalizing problems more fully with a structured interview, additional parental report measures and behavioral assessments. The parents of these twins will be screened for current psychopathology and to collect systematic family history data. The environment of these twins will be studied in the hope of identifying features associated with risk. We will be able to estimate the genetic influence on (a) the measures of the risk factors; (b) the disorders themselves; and (c) the association between the risk factors and disorders. For instance, if temperamental shyness proves to be a risk factor for internalizing disorders of childhood, we can ask (a) if association between shyness and internalizing disorder is genetically mediated; (b) if the disorder can be viewed, genetically, as simply an extreme of the shyness dimension; or, alternatively, (c) whether the association of the shyness and internalizing disorder is environmentally mediated, which would then encourage a search for environmental risk factors/ We will also consider then encourage a search for environmental risk factors. We will also consider possible risk-reducing factors related to resiliency and adaptability, such as the capacity to experience and express pleasure.